This invention relates to a calorimeter for converting a very small amount of heat into an electrical signal and, more particularly, to a radiation detector that provides improved energy resolution and count rate by the use of a superconducting transition edge.
A calorimeter is an instrument for converting externally applied heat into an electrical signal such as current or voltage. Attempts have been made as one example of its application to detect the energy of radiation as a quite small amount of heat. A piece of literature on this is described, for example, by S. H. Moseley et al. in Journal of Applied Physics, 56, 1275 (1984). A calorimeter has a membrane for controlling the flow rate of heat. An absorber (semiconductor) and a thermometer (semiconductor) are mounted on the membrane. The absorber absorbs heat, while the thermometer converts the heat generated by the absorber into an electrical signal. The calorimeter that detects radiation has a quantization efficiency close to 100%. That is, nearly 100% of the energy of radiation is converted into an electrical signal and so there is the advantage that the efficiency is high (i.e., no waste results). Because of this superiority, the calorimeter is adapted as a radiation detector. The energy resolution of a calorimeter is dominated by phonon noise in devices. The effects of the noise can be reduced by using cryogenic temperatures (e.g., below 1 K). In this way, attempts have been made to enhance the energy resolution.
The response speed (i.e., the time between the instant when radiation enters the calorimeter to thereby produce pulses and the instant when the original stable state is regained) of pulses produced by radiation is given by C/G where C is the heat capacity of the calorimeter and G is the thermal conductance of the membrane to dissipate heat to the outside. This conductance indicates the time in which the heat produced by the calorimeter is transmitted through the membrane and escapes.
In 1995, a superconducting calorimeter (hereinafter referred to as a xe2x80x9cTES (transition-edge sensorxe2x80x9d) was reported in which self-feedback function is given at the superconducting transition edge to thereby provide higher energy resolution and higher count rate than the prior art calorimeter (K. D. Irwin, Applied Physics Letters, 66, 1998 (1995)).
With respect to superconductivity, a transition is made from normal conduction to superconductivity at transition temperatures, as shown in FIG. 5. This range of temperatures at which the transition is made is referred to as the superconducting transition edge. The superconducting transition edge is characterized in that a greater amount of change in the resistance occurs for a given amount of temperature change. When heat enters from the outside, the TES produces a quite small change in the temperature. As a result, a greater change in resistance is obtained. When the TEM is driven at a constant voltage at the superconducting transition edge, radiation is absorbed, accompanying a change in the resistance value of the TES. An electrical current corresponding to the change in the resistance is produced. A 1-to-1 relationship exists between the energy of radiation and the peak value of the signal current. The energy of radiation impinging on the TES can be detected by reading the peak value. The self-feedback function permits heat generated inside the TES (active electrons) to escape more quickly than conventional. Higher speed operation of the calorimeter is enabled. Furthermore, the self-feedback function reduces noise and achieves higher energy resolution. Another feature is that the material of the TES can be made entirely from a metal or metals. As a result, the thermal capacity and the electron diffusion time can be reduced. In examples reported heretofore, 200 to 300 xcexcs are reported in the case of an energy resolution of 4.5 eV.
The TES is a calorimeter which makes use of a superconducting transition edge and to which a self-feedback function is given. The TES achieves higher energies and higher speeds compared with calorimeters using semiconductors. It has been difficult to set the fall times of pulses produced in the TES due to radiation to less than 100 xcexcs for the following reason.
The energy resolution of a calorimeter is determined by the variation in the peak values of pulses. Before the temperature in the TES is uniformly elevated by active electrons produced by X-rays, the active electrons are diffused to the outside of the TES. The variation in the peak values increases, thus deteriorating the energy resolution. The process step in which the active electrons uniformly elevate the temperature inside the TES is related to the rise time of pulses. The process step in which the active electrons diffuse out of the TES is related to the fall time. Therefore, in order to improve the energy resolution, the time (xcfx840=C/G) in which active electrons are transmitted through the membrane and escape must be prolonged compared with the rise time of pulses. In this equation, C indicates the heat capacity of the calorimeter and G indicates the thermal conductance of the membrane. Where the rise time is set to 1 xcexcs, for example, it is better to set the xcfx840 to equal to or greater than 1 ms. To improve the count rate, it is necessary to let active electrons escape to the outside as quickly as possible for regaining the original steady state after the calorimeter is elevated in temperature uniformly. That is, it is necessary to shorten the pulse fall time. The count rate is the inverse of 4 times the pulse time constant and indicates the number of pulses capable of being counted per second. Where one wants to improve the energy resolution, a multiple greater than 4 times may be selected.
A calorimeter having a superconducting transition edge has succeeded in shortening the pulse time constant to xcfx84=xcfx840/(1+A) (where A is the feedback constant) by imparting a self-feedback function to the prior art calorimeter. The great advantage of this method over a calorimeter using a semiconductor is that the time in which electrons are caused to escape to the outside by self-feedback after temperature elevation is improved, though the time in which heat is transmitted through the membrane and dissipates is longer compared with the pulse rise time.
From these considerations, it is important to: (1) set the time in which heat is transmitted through the membrane and escapes to be a sufficiently large value compared with the electron diffusion time; and (2) to let electrons inside the calorimeter escape to the outside as quickly as possible after temperature elevation, in order to shorten the pulse time constant of the calorimeter.
A calorimeter using a superconducting transition edge has the problem that if the energy resolution is set to less than 10 eV, the feedback constant A becomes less than 100. Consequently, it has been difficult to set the pulse time constant to less than 100.
A calorimeter of the present invention using a superconducting transition edge and having an absorber for absorbing radiation and producing heat, the absorber being formed on a resistor whose resistance value is varied by the heat. The resistor is formed on a membrane for controlling escape of the heat. The calorimeter is characterized in that it is equipped with a device for letting active electrons produced in the calorimeter escape to the outside.
As a result, if the time (C/G) in which heat produced by the absorber is transmitted through the membrane as phonons and allowed to escape is set long to improve the energy resolution, active electrons inside the TES can be forced to the outside of the calorimeter. Therefore, the pulse time constant can be shortened. In consequence, higher-speed operation of the calorimeter can be realized. Furthermore, the time constant (C/G) can be set sufficiently greater than the time in which active electrons produced by the absorber are diffused inside the calorimeter. Hence, variations in the pulse peak values can be suppressed. Higher energy resolution can be accomplished.
The invention also provides a calorimeter characterized in that the device for letting active electrons escape to the outside is an SIN junction device comprising a three-layer structure consisting of parts of the absorber, a thin insulating film, and a superconductor.
Where the absorber is a normal conductor, the three-layer structure consisting of the absorber, thin insulating film, and superconductor is known as an SIN (Superconductor-Insulator-Normal metal) junction. If this structure is used, the response occurring on application of a voltage can be set to 1 xcexcs. Active electrons produced by the absorber can be quickly sent from the absorber to the superconductor as an electrical current. As a result, a device can be fabricated in which the amount of electric charge flowing from the absorber to the superconductor can be adjusted by applying a voltage between the absorber and the superconductor. A calorimeter capable of providing quick response can be obtained.
In addition, the invention provides a calorimeter which makes use of a superconducting transition edge and in which an absorber for absorbing radiation and producing heat is formed on a resistor whose resistance value is varied by the heat. The resistor is formed on a membrane that controls escape of the heat. This calorimeter is characterized in that it is fitted with a thermal conductance adjuster for adjusting the thermal conductance of the membrane.
To improve the response speed of the calorimeter, it is better to increase the thermal conductance, because the response speed is equal to the calorimeter""s heat capacity divided by the thermal conductance (C/G). However, it is necessary to make uniform the thermalization of the calorimeter in order to improve the energy resolution and so it is better to reduce the thermal conductance. In the present invention, the thermal conductance adjuster is provided which reduces the thermal conductance during the process in which the calorimeter is thermalized owing to absorption of radiation and increases the thermal conductance during the process in which active electrons produced as a result of thermalization are quickly released to the outside. As a consequence, variations in the peak values of the calorimeter can be reduced by reducing the thermal conductance at first. A calorimeter having a higher energy resolution can be accomplished. Furthermore, the steady state prior to radiation absorption can be regained by quickly releasing electrons produced by thermalization to the outside. Hence, faster response and higher count rate can be accomplished. Especially, in order to adjust the thermal conductance, it is necessary to adjust the flow rate of heat from the calorimeter to the outside. Where a resistor is used as the thermal conductance adjuster and the temperature of the membrane is elevated by resistive heating, the temperature gradient between the calorimeter and the ambient can be reduced. For this reason, the flow rate of heat from the calorimeter to the outside decreases. The temperature gradient increases if the resistive heating is stopped. The flow rate of heat from the calorimeter to the outside increases. As a result, if a resistor is employed as the thermal conductance adjuster, the thermal flow rate can be adjusted easily. It is possible to return the apparatus to the steady state prior to radiation absorption by quickly releasing electrons produced by thermalization to the outside. Higher-speed response and higher count rate can be accomplished.